This type represents a SURFace.


A surface is a type of geometry (PK_CLASS_surf is a subclass of PK_CLASS_geom).

A surface is of one of the types:

 - PK_BSURF_t:   a B geometry surface
 - PK_PLANE_t:   an infinite plane
 - PK_CYL_t:     an infinite cylinder
 - PK_CONE_t:    a semi-infinite cone
 - PK_SPHERE_t:  a sphere
 - PK_TORUS_t:   a torus
 - PK_OFFSET_t:  an offset surface
 - PK_SWEPT_t:   a swept surface
 - PK_SPUN_t:    a spun surface
 - PK_BLENDSF_t: a blend surface
 - PK_FSURF_t:   a foreign surface